I feel like today many atheist’s reasonings for their beliefs are based in science and scientific discoveries of the past couple centuries. People living in Ancient Rome had no knowledge of evolution, human anatomy (at least to our extent), the earth’s history, space, etc. What other reason than, “sounds like BS” would give a Roman citizen to think otherwise about the Gods?
Your assertion about atheism isn’t quite accurate. Atheism has existed for a long, long time and is by no means a modern movement defined by scientific advancement. A major portion of religious/spiritual/metaphysical thinking is beyond the realm of science, and it cannot be reduced to being “disproved” because of the discovery of the atom or the neutron.
Religion in many societies is not about belief, but about function, behaviour, and community. The role of the clergy in the Holy Roman Empire was to concern itself with the doctrines and the health of the peoples’ souls. For the common man, religion was just something you “did”.
In Rome, the gods were considered as real as anything else, but this wasn’t a belief, it was “common knowledge” because of the role it fulfilled in society. Furthermore, it differed from what many consider to be religious today. It wasn’t a case of making statements about truth and reality, it was making fundamental statements about people and, most importantly, behaviour. If acting in a certain way displeases the gods, that isn’t a question of belief, but a question of language and concept as it applies to behaviour.
In the Buddha’s time in roughly 450BCE north India, atheism was a well known and common doctrine, as was physicalist reductionism. This doctrine linked everything essentially “human” with the body, and linked all knowledge to direct experience. However, we should be careful about linking this with modern empirical science because, for the ancient Indians, experience was of a phenomenological nature, and not linked to an “external world”. In this manner, we can talk about gods and realms and parables as being “real”, yet clearly distinct from the [concept of] the external world.
In SPQR, Mary Beard states that for Roman citizens, the truth of the gods was a given. But again, this shouldn’t mean that they were all deluded or blindly religious. Many would confidently state they had never seen, heard, or interacted with a god, and had no idea how it worked in relation to the formation of the world, or anything else. For all intents and purposes atheism could exist alongside the function of religion.
Ultimately, we have to see religion in this context, not as something of belief, but of function. By way of analogy, much of ancient Buddhism dealt with cosmological theses, but belief is largely irrelevant. It is a concept of behaviour far more than of belief.
To a Roman citizen, religion would likewise be seen as a functional part of life, and the average man on the street might look more than a little puzzled when pressed about what they actually believe. It is something of a modern shift on thinking that closely ties religion to belief. Throughout all of human history, the billions of humans have believes countless things, including something we would today term atheism. But that, more often than not, had little to do with religion as it functioned in their societies.